The officials said the latest engine trial is the third such test in recent weeks using similar technology.

One official said that the initial assessment indicates that the engine technology could possibly be used in an eventual intercontinental ballistic missile.

It was not clear whether the engine would require some adjustment to be used in an ICBM, if it can indeed be used that way at all.

Possession of an intercontinental ballistic missile could allow North Korea to threaten the continental United States. ICBM technology is considered difficult to develop, with advanced rocket design being a necessary step.

The US is also not sure whether North Korea's military is capable of miniaturizing a nuclear warhead so that it could be mounted on a rocket, such as an ICBM, though Pyongyang has claimed this capability and US officials have said that they incorporate that ability in their planning assumptions.

In a separate move, the US announced Monday that the US Marines have deployed F-35Bs to South Korea for the first time as part of an exercise that began Friday.

"This is the first time we have operated the F-35B in the Republic of Korea," Pentagon spokesman US Navy Capt. Jeff Davis told reporters.

The new short takeoff and vertical landing capable stealth strike fighter was only first declared operational in July of 2015.

The deployment is part of a training exercise with the South Korean military, an exercise Davis called "defensive in nature."

"It goes without saying we are committed to defending the Republic of Korea and Japan against any North Korean aggression," Davis added.